test for lease maint config read from config file, and related refactoring

setup.cfg

src/_zkapauthorizer/lease_maintenance.py

src/_zkapauthorizer/model.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_client_resource.py

src/_zkapauthorizer/tests/test_lease_maintenance.py

src/_zkapauthorizer/tests/test_plugin.py